The present invention generally relates to methods of removing layers of organic matters, and more particularly to a method of removing a layer of an organic matter such as a resist which is used as a mask during an ion implantation and is degraded by the ion implantation.
Ion implantation is used in production steps of a semiconductor device. When making the ion implantation, a layer of an organic matter is used as a mask, and a resist is generally used as the organic matter. Recently, some production steps make the ion implantation with a high dosage of 1.times.10.sup.14 atoms/cm.sup.2.
During the ion implantation, the resist is degraded and it is extremely difficult to completely remove the degraded resist by the normal O.sub.2 plasma ashing. A large quantity of residue remains even after the O.sub.2 plasma ashing, and a surface on which the residue exists must be slightly etched to completely remove the residue. As a result, the freedom with which the semiconductor device may be designed is limited by the need to carry out an etching after the O.sub.2 plasma ashing.
On the other hand, the present inventors have proposed a method of removing the resist by a dry process using a plasma which is obtained by exciting a gas which includes hydrogen but includes no oxygen. According to the proposed method, however, the ashing rate is slow even when a 100% H.sub.2 gas is used. For this reason, this proposed method is impractical for industrial applications. For example, the ashing rate is approximately 700 .ANG./min for a positive resist OFPR800 manufactured by Tokyo Ooka Kohgyo Kabushiki Kaisha of Japan when a reactive ion etching (RIE) is carried out using a H.sub.2 gas with a flow rate of 300 cc/min, pressure of 0.5 Torr and an RF power of 300 W.
The H.sub.2 gas is flammable and it is dangerous to use H.sub.2 gas of a high density. But when the density of the H.sub.2 gas is set to 3% or less which is below the explosive limits, the ashing rate with the above described conditions falls to under 100 .ANG./min.
Therefore, there is a demand for a method of completely removing the degraded resist at a high speed without danger.